Plasmids have been identified in several strains of Streptococcus mutans and several species of the genus Lactobacillus. A new lysis technique, originally developed for oral Streptococci, has been extended to the lysis of almost all strains of Lactobacillus, Streptococcus, Pediococcus, Propionibacterium and Actinomyces. Current work with Lactobacillus plasmids is directed at determining the size and relatedness of these plasmids by electron microscopy, DNA-DNA hydridization and restriction enzyme mapping. Other studies are directed at determining the phenotypic properties coded for by this plasmid using various curing techniques. It is of particular interest to determine if these extra-chromosomal elements have played a role in the recent adaptation of microbes to the oral environment.